A magnetic disk drive moves heads radially over rotating disks, positions the heads over specific data tracks accurately, and writes data on and reads data from the disks magnetically.
FIG. 3 is a schematic sectional view of a typical magnetic disk drive. The magnetic disk drive has four disks 12 and eight heads 11. The eight heads 11 are supported by a rotary actuator 13 which is driven by a voice coil motor 14. On each side of each disk 12, a head 11 writes and reads data.
FIG. 4 is a schematic plan view of the magnetic disk drive. The voice coil motor 14 drives the rotary actuator 13 to move the heads 11 radially over the rotating disk 12.
FIG. 5 is a schematic plan view of part of the magnetic disk drive. Special magnetic data to indicate the positions of the heads are written on the disks 12 before the magnetic disk drive is shipped from the factory. The CPU (central control unit) determines the electric power to be given to the voice coil motor 14 to move the rotary actuator 13 and position the head 11 on a specific track 15 accurately. Although the tracks 15 on the disk is indicated by using solid lines in FIG. 5, they are formed magnetically and therefore not visible optically.
FIG. 6 is a plan view of the head of the magnetic disk drive. The head 11 is of a write/read separation type. A composite head consists of a magnetoresistance-effect read element 61 and an inductive write element 62 both laminated on one and the same slider. The MR element, which makes use of the magnetoresistance effect of iron nickel alloy, is widely used as the read element 61. On the other hand, the GMR element, which is non-magnetic layer sandwiched between magnetic layers, is now being put to practical use. Because the MR element is highly sensitive in reading short-wavelength recorded data, it is effective in increasing the recording density of magnetic disk drives. As magnetoresistance-effect read elements, represented by the MR element, is not capable of writing data on a magnetic disk, an inductive element, which consists of minute magnetic poles and coils formed by photo lithograph, is used as the write element 62. The working principle of the inductive element is the same as that of the tape recorder in common use. Most of the magnetic disk drives recently put on the market use the above composite head. Although each of the read and the write element 61 and 62 is represented by a rectangle to indicate its width and position visually in FIG. 6, the rectangle does not represent the actual optical shape of said element. This is also true of the other Figures.
The main feature of magnetic disk drives is that data once written on a disk can be replaced with new data. By writing new data on a data track already holding data, most parts of the old information are masked. Because this writing method, called “direct overwrite method,” requires no erase process, high writing efficiency can be achieved by using this method, which therefore is adopted by all magnetic disk drives.
As shown in FIG. 6, the width of the write element 62 is made smaller than the track pitch so that the write element 62 does not affect the adjacent tracks and, thereby, the reliability of the magnetic disk drive can be secured. While the write element 62 is writing data on the disk, its magnetic field extends beyond its width toward the adjacent tracks. Therefore, if the write element 62 is as wide as the track pitch, it affects an adjacent zone in the adjacent track on each side of the track which it is writing data on. The width of a track including the adjacent zones on both sides of the track is called “erase track width.” As disclosed in the Japanese Unexamined Patent publication No. 59-168905, the width of the read element 61 is preferably made smaller than the write-track width to such a degree as the necessary SN ratio can be secured, the positioning errors in writing and reading taken into account. Thus, by setting the widths of the write and the read element 62 and 61 properly, the risk can be avoided that the write element 62 affects the adjacent tracks, destroying data in them. Furthermore, by doing so, the frequency of retrials during the reading of data can be reduced, which increases the performance of the magnetic disk drive, and a proper SN ratio can be secured, which reduces the bit error rate.
Because magnetoresistance-effect read elements, represented by the MR element, features its excellent sensitivity in reading data, it is effective in raising the recording densities in the rotating direction of, and radially of, the magnetic disk.
On the other hand, because the inductive write element 62 must have at least a sectional area capable of letting through a magnetic flux which is required to reverse the magnetization of the magnetic layer of the magnetic disk. Therefore, reducing the track width is liable to increase the height of the magnetic poles. For example, the magnetic poles with a write-track width of 1.5 μm are as height as 3 μm or more.
On the other hand, if the ratio of the height to the width, or aspect ratio, of a magnetic pole is made large far beyond 1.0, its structural reluctance becomes large and it becomes difficult to align the magnetic anisotropy of the poles in one direction. Thus, the pole becomes less capable of letting the magnetic flux through effectively. If the strength of the magnetic field emitted from the writing gap of the magnetic poles is reduced, the overwrite performance of the write element 62 is reduced. The transition length of recorded magnetization becomes long and the resolution becomes low. The noise elements due to the medium increase and it becomes difficult to secure the necessary SN ratio, of the magnetic disk drive.
In order to prevent the reduction of the write-track width of the poles from reducing the reluctance of the poles, a technique of reducing the sectional areas required of the magnetic poles by using a material of high saturation magnetic flux density and a technique of reducing the sectional areas of only the tops of the magnetic poles are devised. However, technology to produce magnetic materials of which the saturation magnetic flux density is much higher than that of the currently available iron nickel alloy of a high iron content have yet to be established. To reduce the sectional areas of the tops of the magnetic poles requires a complex process with a long tact time, increasing the manufacturing cost.
In accordance with the prior art, there is a lower limit on the width of the magnetic poles of the write element 62. If the write-track width is reduced below 0.7 μm, a sufficient SN ratio cannot be secured. To solve this problem, the Japanese Unexamined Patent publication No. 7-192226 disclosed a technique of making the write-track width of the magnetic poles of the write head larger than the track pitch of the magnetic disk. In accordance with the invention, a track density higher than that obtainable at the above-mentioned lower limit of 0.7 μm on the write-track width may be achieved. However, it is required to make the read track width of the read head extremely smaller than the track pitch. The smaller the read track width of a read head is, the smaller its output is. Thus, in order to raise the track density by using the above technique, it is necessary to develop a technique to improve the read head.
Under the circumstances, it is hoped to develop a technique to raise the recording density of magnetic disk drives by using a write element of which the magnetic poles are wide enough to generate a strong magnetic field and a read element of which the read track is wide and of which the read sensitivity is high and, at the same time, by reducing the width of data tracks.